The present invention relates generally to printing and more specifically to a method and device for registering printing plates.
Drum mountable offset printing plates are widely used in the printing industry. Typically, such plates are flexible to be wrapped around the printing drum of an offset printing press. The plates are typically rectangular in shape, having one row of holes spaced laterally apart along one end of the printing plate and having another row of holes spaced laterally apart along the opposite end of the printing plate. The industry standard for such printing plates has the hole centers uniformly spaced laterally one half inch apart with a nominal hole diameter of 0.160 inches. When mounted on the printing drum, the holes are placed over pin bars and pulled in tension in a longitudinal, circumferential direction around the printing drum as is well-known. The printing plate has an image imposed thereon corresponding to the material to be printed, such as, for example, a wedding invitation. It is extremely important that this printing image be precisely located on the printing plate to avoid misalignment of the printed image on the printing paper.
To achieve this critical alignment, small press registering systems are used. Such systems are typically easels having paper punch type punches. The punches are used to punch holes through both the printing plate and the masking flat. The masking flat provides a negative having the image to be transposed onto the printing plate. Since the alignment of the image on the printing plate is critical, as discussed above, it is critical to locate the masking flat over the printing plate with precise alignment. Prior art registering systems provide alignment of the unpunched printing plate on the easel by using pins insertable through the 0.160 inch holes. Such pins are fixed with respect to the easel and hold the printing plate in position.
Although the industry standard nominal diameter for the laterally spaced holes is 0.160 inches, it has been observed that the true diameters of such holes vary significantly depending upon the manufacture. Thus, the true diameter of such 0.160 nominal diameter holes has been found to range between approximately 0.150 inches and 0.171 inches. Accordingly, with the pin registering systems heretofore used, an unacceptable degree of play existed between an oversized hole and a fixed registering pin. Such play allowed the printing plate to shift with respect to the easel, thereby reducing the precision with which alignment could be made. One way the prior art has attempted to address this problem is by providing modified registering pins. These modified registering pins have a stepped cross-section providing various cross-sectional diameters at various locations along the axial length of the pin. Thus, the pin provides various cross-sectional diameters to account for the variance in actual diameter of the registering holes. Also, frustoconical pins providing an infinite, rather than incremental, number of cross-sectional diameters have been used.
The present invention provides a significant advance over prior art approaches to this problem. It allows for a single pin size which accommodates the full range of actual hole diameters present in the industry. It furthermore provides for positive registering of the printing plate with respect to the easel in a predetermined location with a high degree of precision. Furthermore, the present invention provides a method and device with which a printing plate may be laterally centered on the easel depending on the lateral location of the row of registering holes with respect to the longitudinal edges of the printing plate.
The present invention achieves these advantageous results by providing a pair of pin members in two of the registering holes and imparting tension in the printing plate along the row of holes by exerting lateral force against the printing plate inside the holes. Accordingly, the present invention is a significant advance in the art.